Servomotor recording system



y 14, 1957 c. F. ROBINSON 2,792,542

SERVOIOTOR RECORDING SYSTEM Filed Aug. 21, 1953 AMPL IF IE I? IN VEN TOR. CHA RLES E ROBINSON ATTORNEY United States Patent SERVOMOTOR RECORDING SYSTEM Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application August 21, 1953, Serial No. 375,787 3 Claims. (Cl. 318-28) This invention relates to a recording system and more particularly to a self-calibrating recording system adaptable to standardization at any desired predetermined level.

In many recording applications it is the overall sensitivity of the system with which the recorder is associated and not just that of the recorder alone which is of importance. Also, it is frequently desirable to standardize such systems to a predetermined level and to even vary the level of standardization from time to time. One example of such a system is a continuous monitoring mass spectrometer in which it is desired to compare the concentration of one or more components in a process stream with the known concentration of the same component or components in a standard sample.

This invention contemplates in a resistance bridge type recorder having a first potentiometer connected between adjacent bn'dge arms, and a variable resistor connected between opposite bridge terminals, the combination comprising a second potentiometer connected in parallel across the first potentiometer, and means for adjusting the setting of the variable resistor to balance the bridge at any desired setting of the second potentiometer.

More specifically the recording system of the invention comprises a resistance bridge, first and second parallel coupled adjustable tap potentiometers connected between adjacent bridge arms as one bridge terminal, a voltage source and a serially connected adjustable tap variable resistor connected across the bridge between second and third bridge terminals, a switch having a switch arm and separate contacts connected to each of the taps of the first and second potentiometers, an amplifier connected to the fourth bridge terminal, the input signal to be recorded being connected between the switch arm and the amplifier, a motor connected to be driven as a function of the output of the amplifier, a clutch connected to separately connect the motor to drive the taps of the first potentiometer and the variable resistor, and means operating the clutch responsive to the setting of the switch.

The second potentiometer in the above described apparatus may be set at any value with the switch connected to the adjustable tap thereof, whereupon the motor drives the tap of the variable resistor through the clutch and responsize to any unbalance output of the amplifier to balance the bridge at the particular setting of the second potentiometer. When a signal representing an unknown parameter to be measured is applied through the switch to the tap of the first potentiometer, the resultant unbalance of the bridge again actuates the motor through the amplifier to adjust the tap of this first potentiometer, the clutch being selectively actuated for this purpose by the change in switch setting. The excursions of the tap of the first potentiometer are recorded as the value sought.

The invention will be more clearly understood with reference to the following detailed description taken in conjunction with the accompanying drawing which is a schematic circuit diagram of a recorder in accordance with the invention.

Referring to the drawing, the recorder comprises the four element resistance bridge including arms 1, 2, 3, 4, each including respective resistance elements R1, R2, tRs, R4. A voltage source 5 and a variable resistor 6, having an adjustable tap 6A connected to tap off a variable portion of the total resistance, are connected between opposite terminals of the bridge formed respectively at the junction of bridge arms 1 and 4 and bridge arms 2 and 3. A potentiometer 7 having an adjustable tap 7A is connected between bridge arms 1 and 2, the tap 7A constituting one terminal of the bridge. A second potentiometer 8 having an adjustable tap 8A is connected across the first potentiometer 7, the two potentiometers being connected in parallel between the two identified bridge arms.

A switch 10 includes a switch arm 10A connected to terminal 11 of a pair of input terminals 11, 12 and selectively to one or the other of contacts S1 and S2, switch contacts S1, S2 are connected respectively to adjustable taps 7A, 8A of the potentiometers 7 and 8. An amplifier 14 is connected at its input to the fourth bridge terminal formed by the junction of arms 3 and 4 and to the other input terminal 12. A motor 16 is connected to the output of amplifier 14 and is driven responsive to the output of the amplifier.

Motor 16 includes a drive shaft 16A connected to a double-clutch, indicated generally at 17. The clutch includes a drive member 17A keyed to the shaft 16A and movable into engagement with one or the other of two driven members 1713 and 17C, which are respectively connected to the tap 7A of the potentiometer 7 and the tap 6A of the variable resistor 6. The drive member 17A is linked to the switch arm 10A, whereby the clutch drive member 17A is shifted into engagement within one or the other of the driven members 17B and 17C. When the switch is connected through contact S1 the clutch connects motor drive shaft 16A to adjustable tap 7A of potentiometer 7, and when the switch is connected through contact S2 the clutch is selectively controlled to connect motor drive shaft 16A to tap 6A of variable resistor 6. A conventional strip chart recorder 20 including a pen 21 is mechanically coupled to adjustable tap 7A of potentiometer 7 so that the pen records the excursions thereof.

The operation of the recorder as illustrated and described, as might be employed to sense and record the output of a process monitoring mass spectrometer, is as follows: With input switch 10 in the S1 position, the signal to be measured is applied across the bridge between potentiometer tap 7A and amplifier 14. Any variation in the input signal varies the bridge balance producing an unbalance voltage at the amplifier input. This unbalance voltage, as amplified, drives motor 16, which, in the indicated switch position, is connected through clutch 17 to tap 7A to vary the setting of this tap to return the bridge to balance. Any variation in the setting of the tap 7A is recorded on chart 20. Bridge balance is, of course, a function of the setting of tap 6A of the variable resistor 6.

As is conventional in the operation of a process monitor mass spectrometer, a so-called standard gas containing the components of interest in known concentration is periodicaliy admitted to the instrument. With the recording system of the invention, the input switch 10 is thrown to the S2 position during such standardizing operation, whereby a mechanical linkage between clutch 17 and tap 6A of variable resistor 6 is completed. Potentiometer 8 has been, or is, manually set at a point corresponding to the indication which the instrument is desired to show on the standard gas in question. The difference between the mass spectrometer signal developed with the standard gas and the output of potentiometer 8 appears across the amplifier 14 and through the servo motor linkage drives tap 6A to cause the difference between the two signals to be substantially zero. By this means the sensitivity of the recorder is adjusted to compensate for changes in sensitivity of the entiresystem including the mass spectrometer.

By means of the parallel coupled potentiometer 8, standardization can be efiectuated to any predesired level whereby the recorded trace can be made to fall at any desired place on the chart for any given value of the parameter to be measured.

Although described in its operation in conjunction with a mass spectrometer, the system is applicable to any recordable situation in which it is desired to standardize to a presentlevel including temperature measurement, rate of flow, gravity, an the like, and has the very important feature of compensating for sensitivity changes in the recorder as well as in the system sensing the param eter under investigation.

This system of recording can be extended to a multiple channel instrument if desired by the inclusion of additional parallel coupled potentiometers, i. e. further duplications of potentiometer 8, and by parallel connection of a plurality of variable resistors replacing the single resistor 6 as illustrated. Such multi-channel operation requires a clutch operable to make the selective interconnections necessary. Duplication of the identified elements and as requiring a clutch of increased versatility is an obvious expedient from the illustrated and described single channel embodiment.

As an example of the relative resistance values of the variable elements of the illustrated recorder, potentiometers 7 and 8 may each have a resistance of about ohms, and variable resistor 6 may have a total resistance of about 500 ohms. These values are exemplary only, and are in no way limiting.

I claim:

1. In a resistance bridge circuit having a first adjustable tap potentiometer connected between adjacent bn'dge arms with the tap of the potentiometer forming one bridge terminal, an adjustable source of potential connected between second and third opposite bridge terminals, an input circuit, am amplifier coupled to receive the difierence between the signal applied to the input circuit and the output signal provided by the bridge circuit, and motor means coupledto the output of the amplifier for controlling the position of the tap of the first potentiometer to cause the difierence between the signal applied to the input circuit and the output signal provided by the bridge circuit to be substantially zero, the improvement which comprises a second adjustable tap potentiometer connected in shunt with the first potentiometer, means for disconnecting the tap of the first potentiometer and for connecting the tap of the second potentiometer to the input circuit, and means for coupling the output of the motor means to control the potential provided by the adjustable source of potential when the tap of the second potentiometer is connected to the input circuit, to

cause the difierence between the signal applied to the input circuit and the output signal provided by the bridge circuit to be substantially zero when the tap of the second potentiometer is connected to the input circuit.

2. In a resistance bridge type recording system having a first adjustable tap potentiometer connected between adjacent bridge arms with the tap of the potentiometer forming one bridge terminal, an adjustable source of potential connected between second and third opposite bridge terminals, an input circuit having first and second terminals for receiving an input signal, an amplifier have one input connection coupled to the second terminal of the input circuit and having its other input connection coupled to the fourth bridge terminal, and a motor coupled to the output of the amplifier, the improvement which comprises a second adjustable tap potentiometer connected in shunt with the first potentiometer, switching means for selectively connecting the first terminal of the input circuit to the tap of the first potentiometer or to the tap of the second potentiometer, and means for coupling the output of the motor to control the position of the tap of the first potentiometer when that tap is connected to the first terminal of the input circuit and for coupling the output of the motor to control the potential provided by the adjustable source of potential when the tap of the second potentiometer is connected to the first terminal of the input circuit.

3. In a resistance bridge type recording system having a first adjustable tap potentiometer connected in series between adjacent bridge arms with the tap of the potentiometer forming one bridge terminal, a voltage source and an adjustable resistor connected in series across the bridge between second and third bridge terminals, an input circuit having first and second terminals for receiving an input signal, an amplifier having one input connection coupled to the second terminal of the input circuit and having its other input connection coupled to the fourth bridge terminal, and a motor coupled to the output of the amplifier, the improvement which comprises a second adjustable tap potentiometer connected in shunt with the first potentiometer, switching means for selectively connecting the first terminal of the input circuit to the tap of the first potentiometer or to the tap of the second potentiometer, and means coupled to the switching means for coupling the output of the motor to control the position of the tap of the first potentiometer when that tap is connected to the first terminal of the input circuit and for coupling the output of the motor to control the resistance of the adjustable resistor when the tap of the second potentiometer is connected to the first terminal of the input circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,891 Schmitt July 30, 1946 2,446,l53 Belcher July 27, 1948 2,489,999 Cherry NOV. 29, 1949 2,527,207 Barnes Oct. 24, 1950 2,657,349 Williams Oct. 27, 1953 2,659,848 Mouzon Nov. 17, 1953 

